In the context of turbine engines, turbochargers use heat and volumetric flow of engine exhaust gas to pressurize or boost an intake air stream into a combustion chamber. Specifically, exhaust gas from the engine is routed into a turbocharger turbine housing. A turbine is mounted inside the housing, and the exhaust gas flow causes the turbine to spin. The turbine is mounted on one end of a shaft that has a radial air compressor mounted on an opposite end thereof. Thus, rotary action of the turbine also causes the air compressor to spin. The spinning action of the air compressor causes intake air to enter a compressor housing and to be pressurized or boosted before the intake air is mixed with fuel and combusted within the engine combustion chamber.
Solid lubricants are useful for reducing the weight and complexity, and increasing the reliability of various high temperature machinery such as turbochargers. Some examples of turbocharger components that would benefit from solid lubricants include the turbocharger shaft and the bearings that support the shaft. Some known solid lubricants include graphite fluoride/polymer composites, molybdenum disulfide, silver, calcium fluoride, and barium fluoride. However, these materials may not be particularly useful for high temperature machinery since they have relatively high porosity, can be difficult to coat, and/or are only stable at temperatures below about 250° C.
Some high temperature airfoil coatings are also useful as solid lubricants. Examples of such airfoil coatings include hard nitride or carbide coatings, and metal-ceramic composite coatings that include embedded solid lubricants such as silver and calcium fluoride, and further include high temperature low-friction materials such as molybdenum sulfide. However, none of the existing coating materials or systems has an adequate friction coefficient, wear property, thermal stability, and load carrying capability over an operating temperature range for many turbocharger components and other high temperature machinery.
Hence, there is a need for methods and materials for manufacturing and/or coating turbomachinery or other machinery with a solid lubricant that has a low friction coefficient and a low wear rate against hard superalloy materials. There is a particular need for a self-lubricating or in situ lubricating material with ceramic-like oxidation resistant properties and metal-like fracture toughness, particularly at high temperatures.